What is agnatic ancestry?

While reviewing Y-haplogroup information it is always important
to keep in mind the profound difference between ordinary ancestry and
agnatic ancestry.
You have two grandfathers, four different great-grandfathers,
eight 2-g grandfathers and so on.
You have over a thousand 9-g grandfathers, over a million
19-g grandfathers, a billion 29-g grandfathers, and over a
trillion 39-g grandfathers!

Ooops! That's impossible; the number of humans who've ever
lived is less than a trillion. You have a trillion 39-g grandfather
slots in your pedigree, but many of these slots will be filled
by the same people. Charlemagne might appear a billion times in your
pedigree!

And yet, among the thousand 9-g grandfathers, only one is
your agnatic 9-g grandfather, your father's father's father's father's
father's father's father's father's father's father's father.
Among the trillion 39-g grandfathers, only one is
your agnatic 9-g grandfather.
Among the googols of 340-g grandfather slots, only one is
the agnatic slot, the ancestor from whom your father
inherited his Y-chromosome.

Indfhine and other Gaelic inheritance rules

In order to determine eligibility and settle claims
(e.g. blood money for murdered kinfolk or ascent to a lordship)
the earliest Irish and Scots people with written records show
that they memorized agnatic genealogies, and we can infer that
they're bards and Druids memorized these lineages long before
there were written records!
There seem to be traces of similar systems in ancient Hindu
culture.
The agnatic lineage is important in some cultures.
And, while male invaders were often happy to mate with indigenous women,
the genetic evidence shows that indigenous males sometimes had
limited procreative success after an invasion.

Ancient Agnatic Lineages

Here's a (very ancient!) genealogical lineage you may be familiar with.

G11: Adam, husband of Eve & 11-gt grandfather of Eber

G10: Seth

G9: Enos, 9-gt grandfather of Eber

G8: Cainan

G7: Mahalaleel

G6: Jared, 6-gt grandfather of Eber

G5: Enoch

G4: Methusaleh, 4-gt grandfather of Eber

G3: Laamech

G2: Noah NOAH:
→ Japheth, Ham

G1: Shem NOAH:
→ Aram, Asshur, Elam, Lud

G0: Arphaxad, grandfather of Eber

G-1: Salah

G-2: Eber the Prophet EBER:
→ Joktam

G-3: Peleg

We ignore mother connections in the genealogies on this page,
so twenty generations get condensed into a very short list.
Adam had multiple sons, but the agnatic lines of all but Seth
eventually went extinct. Similarly, all surviving human males
have the Y-chromosome of Enos, and of Cainan, and indeed of all the
males in the list until we get to Noah.
Eber had sons other than Joktam and Peleg,
by the way, but as far as can be found in
Biblical genealogies, the agnatic lines of the other sons soon went extinct.

If we accept Biblical myth, then Adam is the agnatic ancestor of
ALL living humans, as is Seth, Enos, etc. all the way to Noah.
No other Y-chromosomes have survived. We're all descended from
Noah's uncle Eliakim also, of course -- he was the father-in-law
of Noah's sons, but Eliakim's agnatic went extinct in
the Flood.

Noah had three sons each with living agnates. Men that have three
or more sons with agnates living today are of special interest;
we will label them NOAH,
in acknowledgemnt of the first such (legendary) man.
For each NOAH, all but one of the sons with living agnates apppears
to his right in red (as Japheth and Ham do), while the son whose line
we follow (Shem) appears below the NOAH.
Noah's son Shem also fits the requirement for
"NOAH".
We will also speak of "EBER"s,
who have two sons with living agnates,
again named after a famous Biblical progenitor.

The Y-chromosome of King James I (and VI)

But we're not interested in the legendary ancestry of the legendary
Prophets of Genesis.
Instead we will focus on a particular Stuart family.
And yet we will see that, like the line to the Prophet Eber, there are many
ancestors along the way whose Y-chromosome survives in only a single son.

Stewart is a common surname in the British Isles and America.
There are various different Stewart families, but many are descended
from the ancient Lord High Stewards of Scotland -- it is that line we
will consider.

We will root the tree at James Stuart (King James I of England).
He has many living agnates (although many developed surnames like FitzRoy).
The lineage has a clear character, albeit one which might be viewed in
"racist" terms: We see the a single line of males "walking point", almost
acting as "the tip of a spear."

By the way, 30 years is a good estimate, at least in historic times,
of the generation gap between father and son. (24 years is a good
estimate for mother-to-daughter gaps.)
Here is the lineage of interest.

G10000: b. 298k BC, 10000-gt grandfather of King James
G10000 was the agnatic ancestor of ALL of us.
Even if Homo sapiens existed this early, there's no guarantee
G10000 was of that species. He might have been some other strain
of Homo heidelbergensis
(the common ancestor of Neanderthal man and us),
with his descendants eventually breeding with Homo sapiens
much later.

G8853: b. 264,000 BC Mrca of A00-T EBER→ A00-AF6 (Bangwa & Mbo)
G8853 had two sons,
each with agnates living today. Almost all living humans
are agnates of one son (G8852, aka A0-T) but the very rare Y-chromosome
of a second son has shown up, especially among certain
tribes of Cameroon (and neighboring Berbers).
And did G8853 live in Northwest Africa? That's where most of
his agnates live today and Google finds websites that assume he
lived in Northwest Africa, but a lot of migration can take
place in 260,000 years!
The figures we show (King James' 8853-g grandfather, born
about 264,000 BC was the 'EBER' with two sons who left
agnates) is just an estimate.
Yfull.Com (2017), which we use for other estimates, shows the
A00 branching at 234,000 BC.

G6650: b. 198k BC, 6650-gt grandfather of King James
We know that 99% of living males share the Y-chromosome of G7050,
while the chromosome of A00-AF6 barely survives.
But what were the sizes of these clans 200,000 years ago?
The A00 clan might have been dominant
then; we just don't know. Eventually the DNA in some very old skeletons
may be sequenced and we will know more.

G5550: b. 165k BC, 5550-gt grandfather of King James

G5360: b. 159,300 BC Mrca of A0-T EBER→ A0-P305 (Bakola Pygmies)

G4430: b. 131,400 BC Mrca of A1 EBER→ A1a-M31

G4340: b. 128,700 BC Mrca of A1b EBER→ A1b1-A6 (Bushman etc.)

G3550: b. 105k BC, 3550-gt grandfather of King James

G3050: b. 90k BC, 3050-gt grandfather of King James

G2917: b. 86,000 BC Mrca of BT EBER→ B-M60 (Pygmy & Hadzabe etc.)

G2650: b. 78k BC, 2650-gt grandfather of King James
Mutations facilitating language probably emerged about this time
and soon spread to all human populations.

G2455: b. 72k BC, approx. date of Toba super-eruption

G2350: b. 69k BC, 2350-gt grandfather of King James

G2267: b. 66,500 BC Mrca of CT EBER→ DE G2267 probably lived near Yemen or Ethiopia.
D (to East Asia) & E (back to Africa) soon split.
D is now rarish, found especially in Tibet and among Ainu.
E dominates Africa; especially important are two clades of E,
which we will call E-Berber and E-Bantu, dominant in North Africa
and Sub-saharan Africa respectively.
A few DE* are found in Ethiopia and Tibet.
(There are some E's in the Middle East, and Southern and Southeast Europe
as a result of post-Neolithic migrations. E's are also found
among the Paleolithic skeletons from Europe.)

G2180: b. 63,900 BC Mrca of CF EBER→ C C become dominant in Asia. Today, clades are found in Japan,
Eastern and Northern India, New Guinea and its neighbors,
aboriginal Australia. Another clade is dominant among the
Mongols (and other Asian tribes) and among American tribes like the Apaches.
The C Y-chromosome may have been dominant among the Cro-Magnon (Aurignacian)
of Europe but is very rare in today's Europe.
About this time the bow and arrow appeared in Africa.
(Bone sewing needles to stitch animal hides together may have first
appeared about this time, but weren't well represented until
the Aurignacian era, 30,000 years later.)

G1602: b. 46,500 BC Mrca of HIJK EBER→ H (India) F1, G, H are common among ancient European skeletons, but today
H is almost confined to India (and Romani "gypsies"); G has high levels
only near the Caucasus.

G1558: b. 45,200 BC Mrca of IJK EBER→ IJ After many thousands of years, IJ splits
into I (Europe's Solutrean culture) and J (Near East).

G1550: b. 45k BC, 1550-gt grandfather of King James
The lineage moves Eastward.

G1498: b. 43,420 BC Mrca of K2 NOAH→ NO, K2c, K2d The O haplogroup is dominant in today's East Asia;
N remained single-line until 20,000 BC but is now found throughout
Northern Eurasia, from tribes like Eskimos and Evenks in the East
and Finns and Saami in the West.

G1497: b. 43,400 BC Mrca of K2b EBER→ MS M and S are found in New Guinea and neighboring islands.
The ancestor of P began moving back toward Pakistan, and eventually
northward to the Eurasian steppes.

G1350: b. 39k BC, 1350-gt grandfather of King James

G1150: b. 33k BC, 1150-gt grandfather of King James

G1050: b. 30k BC, 1050-gt grandfather of King James
By this time the Aurignacian (Cro-Magnon) revolution was underway.
But those humans weren't in P haplogoup.
The ancestor of Q and R at this time was just a single man,
somewhere near central Eurasia.

G1037: b. 29,600 BC Mrca of P EBER→ Q Q is rare and scattered over Northeast Asia and India. One branch of Q
bridged across Beringia and eventually populated the Americas.

G950: b. 27k BC, 950-gt grandfather of King James

G927: b. 26,300 BC Mrca of R EBER→ R* With the Aurignacian well underway in Europe, with C, F*/G/K*, and I
haplogroups, a line of R develops in Central Asia.

G924: b. 26,200 BC Mrca of R-Y482 EBER→ R2 (India)

G750: b. 21k BC, 750-gt grandfather of King James

G743: b. 20,800 BC Mrca of R1-P238 EBER→ R1a-M420

G663: b. 18,400 BC Mrca of R1b-M343 EBER→ R1b*

G613: b. 16,900 BC Mrca of R1b-L278 EBER→ PH155

G553: b. 15,100 BC Mrca of R1b-L754 EBER→ V88 V88 is an important clade, traveling to West Africa where it
is associated with the Chadic language family.
It is also found in Europe, etc.

G550: b. 15k BC, 550-gt grandfather of King James

G543: b. 14,800 BC Mrca of R1b-L389 EBER→ L389*

G450: b. 12k BC, 450-gt grandfather of King James

G427: b. 11,300 BC Mrca of R1b-P297 EBER→ M478

G350: b. 9000 BC, 350-gt grandfather of King James

G250: b. 6000 BC, 250-gt grandfather of King James
R1b-P297 is found in the Early Samara culture near the Volga River.

G200: b. 4500 BC, 200-gt grandfather of King James
R1b, R1a and Q are all found in the Pontic-Caspian steppes.

G9: b. ca 1246 John of Bonkyll (2nd in command under Wm. Wallace at at Battle of Falkirk)NOAH→ James of Pierston (ancestor of Earls of Atholl etc.), Alexander (ancestor of Earls of Angus), etc. The S781 mutation is known to have occurred in John of Bonkyll specifically.

Quoted from Nichol (Gaelic and Gaelicised Ireland during the Middle Ages)

One of the most important phenomena in a clan-based society is that of
expansion from the top downwards. The seventeenth-century Irish scholar and
genealogist Dualtagh Mac Firbisigh remarked that 'as the sons and families of
the rulers multiplied, so their subjects and followers were squeezed out and
withered away; and this penomenon, the expansion of the ruling or dominant
stocks at the expense of the remainder, is a normal feature in societies of this
type. It has been observed of the modern Basotho of South Africa that 'there is
a constant displacement of commoners by royals [i.e. members of the royal clan]
and of collateral royals by the direct descendants of the ruling prince;, and
this could have been said without adaptation , of any important Gaelic or
Gaelicized lordship of late medieval Ireland.

In Fermanagh, for example the kingship of the Maguires began only with the
accession of Donn Mór in 1282 and the ramification of the family - with the
exception of one or two small and territorially unimportant septs - began with
the sons of the same man. the spread of his descendants can be seen by the
genealogical tract called Geinelaighe Fhearmanach; by 1607 they must have been
in the possesion of at least three-quarters of the total soil of Fermanagh,
having displaced or reduced the clans which had previously held it. The rate
which an Irish clan could itself must not be underestimated. Tulrlough an fhíona
O'Donnell, lord of Tirconnell (d. 1423) had eighteen sons (by ten different
women) and fifty-nine grandsons in the male line. Mulmora O'Reilly, the lord of
East Brefny, who died in 1566, had at least fifty-eight O'Reilly grandsons.
Philip Maguire, lord of Fermanagh (d. 1395) had twenty sons by eight mothers,
and we know of at least fifty grandsons. Oliver Burke of Tirawley (two of whose
became Lower Mac William although he himself had never held that position) left
at least thirty-eight grandsons in the male line.

Irish law drew no distinction in matters of inheritance between the
legitimate and the illegitimate and permitted the affiliation of children by
their mother's declaration (see Chapter 4), and the general sexual
permissiveness of medieval Irish society must have allowed a rate of
multiplication approaching that which is permitted by the polygyny
practised in, for instance, the clan societies of southern Africa already cited.